The present invention relates to a method of driving a liquid crystal display (LCD), especially a simple matrix type LCD, a drive IC for the method and a drive circuit using the drive IC.
An LCD has been widely used in a personal computer, a word processor, and other electronic equipment for its thin and light features, while its display capacity has been increased rapidly. Especially, a super twisted nematic (STN) type LCD is widely used in inexpensive equipment since its cost is lower than a thin film transistor (TFT) type LCD.
An STN type LCD increases its display capacity by increasing a twist angle of a liquid crystal molecule more than two hundred degrees so as to sharpen electro-optical properties of a threshold characteristic of the LCD. The STN type LCD can be manufactured at a low cost compared to a TFT type LCD that has an active matrix structure with a switching element for each pixel.
A multiplex drive method is generally used for driving a simple matrix type LCD including the STN type LCD. The simple matrix LCD has no switching element for each pixel, so that a display intensity of a pixel depends on a root mean square (rms) value voltage including a state in which the scanning electrode of the pixel is not selected. This multiplex drive method keeps display uniformity by making rms voltages equal between enabled pixels as well as disabled pixels.
FIG. 47 shows the above mentioned drive method. Numeral 503 is an LCD panel, 504-507 are scanning electrodes, and 508-511 are signal electrodes. A scanning voltage pulse (+Vs) 501 is applied to the scanning electrodes in order, and signal voltage 502 is applied to the signal electrode, where the signal voltage 502 corresponds to on/off states of the pixel on the selected scanning electrode. The signal voltage is -Vd for the on state and +Vd for the off state. The polarity of the voltage is reversed over a predetermined period to apply an alternating voltage to the liquid crystal.
In a real LCD panel, there is a switching distortion of the voltage wave form applied to the liquid crystal, due to a CR circuit made of an electrode resistance of the scanning electrode and/or the signal electrode, an output resistance of the drive IC and a capacitance of the liquid crystal. Therefore, the rms voltage applied to each pixel deviates from an ideal value, so that the intensity of the pixel, which should be constant, varies depending on a display pattern of other parts. This phenomenon is so-called "crosstalk".
There are several causes of such a crosstalk. The most important and basic cause is a switching distortion of a data signal. In FIG. 47, though only four scanning electrodes 504-507 are shown, there are plural electrodes following the electrode 507, and all pixels are supposed to be in the on state (i.e., white is displayed). For example, the signal voltage applied to the signal electrode 509 is switched three times between off and on states during scanning periods of the scanning electrodes 504-507, while the signal voltage applied to the signal electrode 508 maintains the on state without switching. Therefore, pixels on the signal electrode 509 are provided with a lower rms voltage due to the switching distortion compared with the pixels on the signal electrodes 508. As a result, the white level of the pixels on the signal electrode 509 is darker than that of the pixels on the signal electrode 508, so that stripes are displayed even though the display data are all white. This crosstalk is called a character crosstalk.
In a liquid crystal display, a dc voltage is prevented from being applied to the liquid crystal by switching the polarity of the scanning voltage as well as the polarity of the signal voltage of the data signal in a predetermined period. A drive method for decreasing the character crosstalk is disclosed in Japanese laid open patent application (Tokukai-Sho) 60-19195 and the technical report of Japanese Television Gakkai, IPD82-4 (1983). In this drive method, the switching frequency of the driving voltage polarity is increased in a constant intensity display part by switching drive voltage polarity based on a period of plural horizontal scanning periods that is shorter than one frame. Currently, it is normal to switch the polarity every 10-30 horizontal scanning periods, that is one to several tens of switching frequency per one frame in an LCD having 200-500 scanning lines.
However, this drive method can not eliminate the character crosstalk completely. In addition, this drive method may create another crosstalk (vertical line crosstalk) when a vertical bar is displayed since the polarity switching generates a voltage distortion on the scanning electrode (refer the text of The Second Fine Process Technology Japan '92 Seminar R17).
Another drive method is explained in Japanese laid open patent application (Tokukai-Hei) 4-360192 or 8-292744. This method suppresses the crosstalk by shifting the output level of the signal voltage so as to compensate the switching distortion when the signal voltage switches its level with regard to the non-selected level of the scanning voltage. As shown in FIG. 48, when the output level of the signal voltage is switched, a compensating pulse 521 is added, which shifts the output level of the signal voltage for a predetermined period, so as to compensate for an rms voltage decrease due to the waveform distortion. In this Figure, the non-selected level of the scanning voltage is shifted from V1 to V4 when the polarity of the scanning voltage is switched, for controlling the output voltage of a scanning IC.
FIG. 49 shows a drive circuit for obtaining the wave form shown in FIG. 48 as disclosed in Tokukai-Hei 4-360192. This drive circuit generates four additional voltage levels VDD, V2, V3, V5. An LCD driving voltage generator 525 generates ten voltage levels VDD, VDD', V1-V5, V2', V3' and V5', and eight levels of them are supplied to a signal drive circuit 523. Numeral 522 is an LCD panel and 524 is a scan drive circuit.
If the non-selected level of the scanning voltage is a constant value V1, the signal voltage waveform is as shown in FIG. 50. This is obtained by shifting the latter half of the signal voltage waveform in FIG. 48. The scanning IC is required to output positive and negative pulses (+/-Vs), and the lower half of the voltage level generated by the LCD scan voltage generator 525 is not necessary.
In the drive method disclosed in Tokukai-Hei 8-292744, a compensating pulse is superimposed on the supplied voltage to the signal drive circuit for obtaining the waveform shown in FIG. 48 or FIG. 50. This drive method makes the output of the signal drive IC high impedance so that the compensating pulse does not reach the signal electrode when the signal voltage is not switched (is not inverted), and turn on the output of the signal drive IC so that the compensating pulse is applied to the signal electrode when the signal voltage is switched (is inverted).
Another drive method is disclosed in Tokukai-Hei 5-333315. This drive method adds a pulse voltage that decreases the rms signal voltage when the signal voltage is not inverted, opposite to the above mentioned method disclosed in Tokukai-Hei 4-360192 or 8-292744, so as to generate a waveform distortion that may occur when the level is inverted and makes both rms voltages equal. The non-selected level of the scanning electrode or the opposite level of the signal voltage (the off level when continuing on signal, and the on level when continuing off signal) is used as a compensation voltage level, so that the crosstalk is suppressed without additional voltage levels.
The above mentioned drive methods in the prior art have some disadvantages as explained below.
In the method of Tokukai-Hei 4-360192, the number of the voltage levels supplied to the LCD drive IC is increased, along with the numbers of bus wires and switches in the drive IC as well as the numbers of connections between the drive IC and a power source circuit. The number of the voltage levels supplied to the signal drive IC is increased from four to eight when using the waveform of FIG. 48, and from two to four when using the waveform of FIG. 50, by adding the compensating pulse. Thus, areas of the drive IC and the connecting portion are increased, so that the cost of the IC rises and the area of a peripheral portion of the LCD panel increases.
In the drive method disclosed in Tokukai-Hei 8-292744, while the output of the signal drive IC is in a high impedance state, the signal electrode corresponding to the output is in a floating state, so that the signal electrode discharges. As a result, contrast of the LCD drops, and an uneven display state may occur.
In the drive method disclosed in Tokukai-Hei 5-333315, the level of the compensation voltage is shared with another voltage level, so that the voltage switching width for the compensation is large. In this method, a large voltage switching occurred once during one horizontal scan period when the signal voltage is inverted, and twice for leading and falling edges of the compensating pulse during one horizontal scan period when the signal voltage is not inverted. On the other hand, the drive method without compensation of the crosstalk does not cause the voltage switching when the signal voltage is not inverted. When the number of scan lines is n, the signal voltage switchings occur n-2n times in the drive method disclosed in Tokukai-Hei 5-333315. This number n-2n is much bigger than 0-n that is the number of switching in the drive method without compensation of the crosstalk. The consumption of power also increases along with the number of switchings.
Furthermore, in any drive method mentioned above, the compensation waveform has high frequency components, so that the compensation is not even in the screen, and compensation characteristics may vary depending on a size of the LCD panel, a number of pixels and physical constants of the liquid crystal.
The main purpose of the present invention is to improve the above mentioned drive method in the prior art so that the crosstalk is eliminated or decreased and to suppress increasing of the area of the peripheral portion of an LCD as well as a cost and a power consumption of a drive IC, thus realizing an inexpensive and low-power LCD.